Heart failure has a prevalence in the United States of more than 2 million patients with at least 400,000 new cases diagnosed each year. Mean life expectancy following a diagnosis of cardiac failure is less than 2 years. Heart failure is responsible for at least 200,000 deaths each year, with annual costs estimated to exceed 10 billion dollars.
Most cases of heart failure are caused by left ventricular dysfunction, due to myocardial hypocontractility from coronary ischemia. Other causes of left ventricular failure include non-ischemic conditions such as idiopathic cardiomyopathy or dilated cardiomyopathy due to toxins (such as alcohol and cocaine), infections (such as coxsackievirus, trichinosis, and lyme disease), or infiltration (such as in hemochromotosis and sarcoidosis).
Cardiogenic shock, the end-stage of cardiac failure, occurs in 150,000 patients per year in the United States. Although advances in medical and surgical therapy, e.g., pharmacotherapy and emergency coronary artery bypass grafting surgery, have benefited a few of these patients, the majority are refractory to standard medical and surgical treatment. For these patients, heart transplantation is the only proven treatment. However, fewer than 3,000 donor hearts become available each year. Even in patients accepted for transplantation, a significant waiting list mortality has been observed.
A variety of ventricular assist devices (VADs) have been invented since the development of the first successful case of mechanical assistance over 30 years ago to support patients suffering from acute but reversible ventricular failure and to sustain patients who are candidates for transplantation. The first generation of VADs, developed by the early 1980s, consisted of external pumps connected to a patient's heart by surgically implanted cannulae. The Abiomed BVS 5000 system was widely used for short-term cardiac support as a bridge to transplant in 1992. Other VADs utilizing external pneumatic pumps for short-term support include the Thoratec Ventricular Assist Device and the CardioWest VAD. Disadvantages associated with these external VADs are that (1) multiple large surgical incisions, including median sternotomy and perforation of the ventricle, to connect the support system to the heart are required, (2) the surgical sites pose continuing risk of infection and other complications, such as hemorrhage, (3) the large console needed to house power and control systems severely limits the patient's mobility, thereby requiring hospitalization for the patients.
To overcome the disadvantages associated with the external VADs, pumps small enough to be fully implanted within a patient's body were developed. Two other implantable VADs, the TCI HeartMate and the Novacor Ventricular Assist System, have been developed for long-term use in heart failure patient as an alternative to cardiac transplantation. These devices consist of implantable pumps connected to the left ventricle and the aorta through extracardiac blood conduits. These devices, however, are associated with high complication rates, including (1) substantial morbidity due to the median sternotomy (Nocacor) with cardiac mobilization, often in patients who have had previous cardiac surgery, (2) morbidity due to extensive dissection of the abdominal wall to create a pocket for the pump, (3) bleeding requiring re-operation (30%), (4) bleeding from anticoagulation use, (5) infection (50%) (both nosocomial and device related), (6) thromboembolism, especially to the brain (cerebral emboli were detected using transcranial doppler in two thirds of the patients with ventricular assist devices), and (7) significant bleeding associated with the extensive surgery often results in right heart failure (20%).
The "Acorn device," a metallic envelope which surrounds the ventricles to prevent progressive cardiac dilatation and facilitate ventricular contraction during systole, has been developed to bypass the complications associated with the implantable VADs. Unfortunately, the Acorn device also requires surgical implantation, and its efficacy has not yet been tested in humans.
To reduce morbidity associated with extensive surgical procedures, such as median sternotomy, an endoscopically insertable intraventricular pump (hemopump) is being developed. This device is inserted endoscopically, through the aortic valve and into the left ventricle. The hemopump is capable of delivering blood at 1.5 to 2 liters per minute, with demonstrated mild improvements in hemodynamic parameters. However, peri-procedural morbidity associated with using this device is high, including stroke. Systemic anticoagulation is usually necessary for its insertion and is associated with increased risk of hemorrhage. Failure of device insertion occurs in 25% of patients, partly due to iliofemoral atheromatosis and inability to cross the aortic valve. The hemopump is contraindicated in patients with ascending aortic aneurysms, aortic stenosis, aortic regurgitation, and/or artificial aortic valves. Apart from having high complication rates, an additional problem associated with using the existing VADs is that patients can not be weaned from the VADs without another surgical procedure to remove the implanted devices.
What is needed are simpler and safer methods for treating patients with heart failure refractory to medical therapy without the need for surgical procedures, and with fewer associated complications. Existing devices are inadequate for this purpose.